Hybrid vehicles are equipped with an internal combustion engine(s) and a motor(s) as driving source to generate drive power to run the vehicles. In other words, hybrid vehicles run by transferring torque generated with at least one of the engine(s) and the motor(s) toward a drive shaft(s) connected to drive wheels of the vehicles.
Hybrid vehicles may be equipped with “an internal combustion engine using fuel injection valves that directly inject fuel in combustion chambers (cylinders)”. Such fuel injection valve is hereinafter referred to as “in-cylinder fuel injection valve”, and such engine using the in-cylinder fuel injection valve is hereinafter referred to as “in-cylinder fuel injection type engine”. The in-cylinder fuel injection type engine can decrease temperature in the cylinder due to the direct fuel injection in the combustion chamber. Then, the engine can set ignition timings at a more advanced timing since the decrease of the temperature can prevent knocking of the engine. As a result, output torque and fuel efficiency of the engine can be enhanced.
On the other hand, regarding the in-cylinder fuel injection type engine, fuel adherence to an inner surface of the combustion chamber is likely to occur especially when the temperature of the engine is low. The adhered fuel to the inner surface of the combustion chamber intrudes in a crankcase through a clearance gap between the inner surface and a piston then to be mixed in lubrication oil (engine oil) of the engine. In other words, the lubrication oil is diluted with fuel. The mixed fuel in the lubrication oil evaporates at once when the temperature of the engine increases. The evaporated fuel is returned to an intake system of the engine by a PCV system (Positive Crankcase Ventilation system). As a result, the air-fuel ratio of air-fuel mixture supplied to the engine (air-fuel mixture of the engine) may not be controlled appropriately.
In view of the above, one of conventional technology is designed to change operating points of the engine (the engine rotation speed and the engine load) depending on the amount of fuel mixed in the lubrication oil (hereinafter referred to as “amount of mixed fuel” or “amount of diluting fuel”) so that the amount of mixed fuel does not reach to an excessive amount (for example, see the patent literature 1).